Fabrication of liquid crystal devices

ABSTRACT

To provide a uniform composition of the liquid crystal material throughout the device, the interior surfaces of the device are coated or contacted, prior to filling with the liquid crystal material, with certain additive materials normally contained in the liquid crystal material to improve the device performance.

United States Patent Sorkin et al. 51 Oct. 17, 1972 [54] FABRICATION OFLIQUID CRYSTAL [S6] References Cited DEV ES k k UNITED STATES PATENTS[72] inventors: Howard Sor in, Ber eley Heights;

- Richard Ira Klein, Edison both of 2,433,177 12/1947 Waimo ..l4l/48Primary Examiner-Houst0n S. Bell, Jr. [73] Assignee: RCA CorporationAttorney-Glenn H. Bruestle [21] Appl' N 155,897 To provide a uniformcomposition of the liquid crystal material throughout the device, theinterior surfaces of the device are coated or contacted, prior tofilling :i 'f with the liquid crystal material, with certain additivematerials normally contained in the liquid crystal [58] held of Search 1350/ 1 4 2 5 material to improve the device performance.

4 Claims, 1 Drawing Figure FABRICATION OF LIQUID CRYSTAL DEVICESBACKGROUND OF THE INVENTION This invention relates to the fabrication ofliquid crystal devices. 7

One known type of liquid crystal device comprises a container,electrodes on the walls of the container, and

'a film of liquid crystal material therewithin. By applying a voltagebetween various pairs of electrodes, portions of the liquid crystal filmin the fields of the electrode pairs can be reversibly switched betweenlight scattering and light transparent states.

A fairly large number of liquid crystal materials are known. At leastsome of these materials are rather complex, and contain various additivematerials for achieving certain desired results. For example, in certainliquid crystal materials, one additive material, such asanisylidene-p-aminophenol, is used to improve the alignment capabilityof the material, i.e., the ability of the optical domains of thematerial to properly align with one another to provide the film state oflight transparency, and another additive material, such ashexadecylpyridinium bromide, is used to promote the state of lightscattering when a voltage is applied between the device electrodes.

A major problem in the past has been that of providing the variousadditive materials in the proper quantities where needed within thedevice. For example, it was discovered, after much investigation by theinventors herein, that during the filling of the container with theliquid crystal material, adsorption by the surfaces of the container ofthe various ingredients of the material appears to occur at differentrates, whereby the composition of the liquid crystal material within thedevice varies depending upon the degree of contact of the liquid crystalmaterial with the container surfaces. This results in variations, frompart to part of the device, in the device response and appearance.

Also, while the presence of certain additive materials is necessary toachieve certain desirable characteristics, it often occurs that thesematerials also produce undesirable characteristics. For example, in oneliquid crystal material, the additive material used to promote lightscattering is also effective to lower the electrical resistivity of theliquid crystal material composition. This raises the power consumptionof the device which, in some instances, as when the device is batteryoperated, is undesirable. Thus, in the past, various compromises indevice performance and appearance would often have to be made in orderto achieve a balance between the desirable and undesirable effects of agiven additive material.

DESCRIPTION OF THE DRAWING The drawing is a view, in cross-section, of atypical known type of liquid crystal device.

DETAILED DESCRIPTION OF THE INVENTION With reference to the drawing, aliquid crystal device 10 of known type is shown. The device 10 comprisesa pair of oppositely disposed substrates 12 and 14 maintained inspaced-apart relation by means of a shim l6, e.g., of glass frit. On theinside surface 18 of each substrate l2 and 14 is disposed an electrode20 of, e.g., a transparent material such as tin oxide. Means for makingelectrical connections to each of the electrodes 20 comprise conductivestrips 22 on the substrate surfaces 18 which extend between each of theelectrodes 20 and an outer exposed portion 24 of the substrates 12 and14, where terminal leads 26 are bonded thereto. Disposed between thesubstrates 12 and 14 is a thin film 28 of a liquid crystal material, thethickness of the film 28 being in the order of one-half mil.

Means for fabricating devices of the type shown in the drawing aregenerally known. In the filling of the device with the liquid crystalmaterial, according to one known technique, two tubulations (shown indashed lines in the drawing) are provided at opposite ends of the devicewhich communicate with the interior thereof. The remote end of one ofthe tubulations is immersed in a body of'liquid crystal material, andthe other tubulation is connected to a source of vacuum. By exhaustingthe device, the liquid crystal material is drawn thereinto.

Examples of liquid crystal devices, methods of fabricating and operatingsuch devices, and examples of various liquid crystal materials areprovided in U.S. Pat. Nos. 3,499,l 12, issued to Heilmeier, et al., onMar. 3, 1970, and U.S. Pat. No. 3,499,702, issued to Goldmacher, et al.,on Mar. 10, 1970. The instant invention has utility in the fabricationof a wide variety of liquid crystal devices using various liquid crystalmaterials as disclosed in these two patents.

Of great importance in connection with the instant invention is not onlyour discovery of solutions to the heretofore described problems ofliquid crystal devices, but our discovery of the nature of the problemsthemselves. For example, in the early development of liquid crystaldevices, various satisfactory experimental compositions of liquidcrystal materials were developed. When it came to utilizing various onesof these liquid crystal compositions in commercial devices manufacturedaccording to more mass production techniques, however, it was found thatit was extremely difficult to fabricate devices having uniformcharacteristics over the extent of each device. Thus, in some cases,even without the application of a voltage between the device electrodes,portions of the liquid crystal film were at least partially lightscattering. Conversely, other portions of the liquid crystal film, oftenwithin the same device, refused to become light scattering when thevoltage was applied.

At first we thought that these problems were related to the presence ofcontaminants in the devices. Great efforts were expended in providingcleaner techniques in the preparation of the device containers and theliquid crystal materials. While some improvements resulted, the basicproblems remained.

We eventually discovered that these problems at least appeared to bemost pronounced, or localized, at the interface between the liquidcrystal material and the surfaces of the container contacted by theliquid crystal material. This discovery was not easy to make owing tothe extreme thinness of the liquid crystal films used in the devices.From this determination, and further investigations, we discovered thatthese problems are caused by variations in the composition of the liquidcrystal material along the interface between the liquid crystal materialand the container surfaces. As above noted, we believe that thisvariation in the composition of the liquid crystal material is caused bydifferential adsorption, or stripping, of various ingredients from theliquid crystal material by the container surfaces during the filling ofthe devices with the liquid crystal material. That is, as the liquidcrystal material moves through the small space between the containerwalls during the filling of the container, various ingredients of theliquid crystal material are stripped therefrom with the result that thematerial reaching the areas of the container most remote from the inlettubulation are depleted of certain ones of the ingredients. This problemis especially prevalent in large area devices.

A solution which we finally arrived at is to first thoroughly wet orpre-contact the container surfaces to be contacted by the liquid crystalmaterial in the finished devices with the various ingredients oradditives of the liquid crystal material which tend to be strippedtherefrom, and to thereafter fill the container with the liquid crystalmaterial. Thus, after such initial or pre-contacting of the containersurfaces with the liquid crystal ingredients, in some cases to the pointof saturating the surfaces with these ingredients, upon filling thecontainers with the liquid crystal material, the various ingredientswithin the liquid crystal material are not stripped therefrom.

Various methods are possible to achieve this precontacting operation. Inone method, a preliminary quantity of the liquid crystal material itselfcan be flushed through the container with the result that variousingredients of the liquid crystal material are stripped therefrom todesirably pre-contact or dope the container surfaces. A sufficientlylarge quantity of the liquid crystal material is used whereby a thoroughflushing of the container occurs. Thereafter, a new or fresh batch ofthe liquid crystal material is used to fill the container. Since thecontainer surfaces have been pre-contacted or pre-doped with the variousliquid crystal material ingredients, little or no additional strippingof these ingredients from the fresh batch of liquid crystal materialused in the final filling of the container occurs, whereby thecomposition of the liquid crystal material throughout the container issubstantially uniform.

In another method, a gaseous medium carrying various ones of theadditive materials which tend to be stripped from the liquid crystalmaterial, or a vapor of these additive materials alone, is flushedthrough the cell prior to the filling of the cell withthe liquid crystalmaterial. The additive materials are thus deposited on the interiorsurfaces of the liquid crystalcell.

Another method which can be used involves an improved process describedin a patent application filed. concurrently herewith by Herman Stern,and entitled Fabrication of Liquid Crystal Devices. As described andclaimed therein, a large quantity of the liquid crystal material,several times the volume of the container, is pumped through thecontainer to thoroughly flushout the interior thereof while the walls ofthe container are alternately outwardly and inwardly bowed. In thismanner, the preferred paths of the liquid crystal material through thecontainer are caused -to vary, whereby all the surface portions of thecontainer are thoroughly contacted and wetted by the liquid crystalmaterial prior to the tipping off of the fill tubulations to 4- trap afinal quantity of the liquid crystal material 'within the container.

Another aspect of this invention from which special advantages arederived is now described. In one mode of operation of liquid crystaldevices, such as that described in the aforecited U.S. Pat. No. 3,499,112, light scattering is achieved by causing turbulence of the liquidcrystal material by the movement of charged ions therethrough. The ionsare provided, in some instances,

tive materials is that they reduce the electrical resistivity of theliquid crystal material, thereby increasing the power consumption of thedevice during operation thereof. While this is generally of noparticular problem when the device is operated from'a large source ofpower, in some instances, such as when the devices are operated from abattery, low power consumption for longer battery life is greatly to bedesired.

Heretofore, it was found that unless a minimum quantity of the ionizingadditive material was present in the liquid crystal material, portionsof the liquid crystal device would suffer'from hang-up," i.e., theseportions would not become light scattering when a voltage was appliedthereto. This minimum quantity of the additive material, however, tendedto excessively lower the electrical resistivity of the liquid crystalmaterial, hence increase the power used by the device.

In accordance with the instant invention, we discovered that bypre-contacting the container surfaces with the ionizing additivematerial, the minimum quantity of the additive material required toavoid the hang-up problem is significantly reduced. The reason is asfollows.

According to the prior practice, as heretofore explained, during thecell filling process, some of the ingredients are stripped from theliquid crystal material resulting in a variation in the composition ofthe liquid crystal material throughout the device. By providing (withoutknowing why in the prior art process) a relatively large minimumquantity of the ionizing additive material, the stripping of some ofthis additive material still leaves a sufficient quantity of theadditive material to allow proper operation of the device. That is, evenat the particular location of the cell where the maximum stripping ofthe ionizing ingredient occurs, the remaining quantity of the additivematerial at this location is still sufficiently high for-adequate deviceperformance. At other locations'of the cell, however, where theconcentration of the additive material is somewhat higher, owing to alesser degree of stripping, more of the additive material is presentthan is actually required. This excessive additive material. causes alowering of the electrical resistivity of the liquid crystal material toan extent that is not actually necessary for proper operation of thedevice. Thus, by pre-contacting the container surfaces with the ionizingadditive material, in

accordance with the instant invention, the concentration of the additivematerial in the liquid crystal material used to fill the cell need onlybe that which is necessary to provide proper device operation. That is,an ex cessive quantity of the additive material to make up for amountsof the additive material stripped from the liquid crystal material isnot necessary since the precontacting process of the instant inventionlargely avoids or completely eliminates such stripping action.

Any of the above-described processes for pre-contacting the cellinterior surfaces can be used. Moreover, owing to the pre-coating of thecell surfaces, in some cases, the final quantity of the liquid crystalmaterial in the completed device need contain, as it is introduced intothe device, none of the ionizing material. While not known for certain,it is believed that some of the additive material on the cell interiorsurfaces becomes dissolved in the liquid crystal material, therebyproviding the necessary ions.

Specific examples of the invention are now described. One materialhaving utility as an ionizing additive in various liquid crystalcompositions such as ones disclosed in U. S. Pat. No. 3,499,1l 2 ishexadecylpyridinium bromide. According to one embodiment of thisinvention, a solution of this material (HDPB), comprising 0.0l0.2percent, by weight, of the HDPB, balance isopropanol, is flushed throughthe cell via the two tubulations. The extent of flushing is notcritical, three to five times the cell volume of solution can be used.The excess solution is then pumped out of the cell by drawing a vacuumthrough one of the tubulations.

' The cell is then heated in a vacuum oven at a moderate temperature, inthe range 30 60 C., so as not to decompose the additive material, for3-4 hours. The dried cell is then filled with the liquid crystalmaterial, and the cell sealed off.

While the liquid crystal material used in the above example can containsome of the HDPB, in the order or 0.001 percent, by weight, in someinstances, the liquid crystal material can contain no HDPB at all. Inthe past, obtaining adequate device performance required the use of theHDPB in the liquid crystal material at a concentration in the order of0.05 percent. Using a liquid crystal material containing no I-IDPB, inaccordance with the method of this invention, the electrical resistanceof the cells is increased by a factor in the order of 100 in comparisonwith the prior art devices.

In another embodiment of the invention, a mass of the HDPB is gentlyheated in a closed container to a temperature in the order of 65 C., andthe vapors so produced are passed through the liquid crystal cellmaintained at a temperature of about 95 ,C. Preferably, to obtain moreuniform coating of the cell interior surfaces, the vapor is caused toflow through the cell in one direction, for a period of time in theorder of 15 minutes, and then caused to flow through the cell in theother direction, as by admitting the vapor into the cell through theother tubulation, for about the same period of time.

Comparing the above-described two methods, somewhat better results, withrespect to the uniformity of coating of the cell surfaces, are obtainedusing the liquid carrier method.

The suitability of the various disclosed methods of pre-contacting thecontainer surfaces is dependent upon the particular additive materialsinvolved. The use of a gaseous medium for the pre-contacting process,for example, preferably requires an additive material, such as the HDPB,which has a reasonably high vapor pressure. In the usual case, however,the

medium to convey the additive material in the pre-contacting process canalways comprise the liquid crystal material itself, enough of the liquidcrystal material being flushed through the cell to achieve the desireddegree of pre-contacting.

We claim:

1. A method of fabricating a liquid crystal device comprising acontainer and a preselected liquid crystal material plus an additivematerial for said liquid crystal material within said container, saidmethod comprising:

pre-contacting a surface of said container to be contacted by saidliquid crystal material with said additive material, and

thereafter, filling said container with said liquid crystal material.

2. The method as in claim 1 wherein said pre-contacting step comprisesflowing a medium containing a preselected proportion of said additivematerial through said container to thoroughly flush said container withsaid medium.

3. The method as in claim 2 wherein said flowing step comprises using amedium in which said proportion of additive material therein is greaterthan the proportion of said additive material-in said liquid crystalmaterial.

4. A method of fabricating a liquid crystal device comprising acontainer and a filling including a mixture of a liquid crystal and anadditive material, said additive material tending to be taken up by thewalls of said container and thereby depleted from said mixture, saidmethod comprising:

flushing said container with additive material; and

then

filling said container with said mixture;

said flushing step serving to prevent depletion of said additivematerial from said mixture placed in said container by said fillingstep.

2. The method as in claim 1 wherein said pre-contacting step comprisesflowing a medium containing a preselected proportion of said additivematerial through said container to thoroughly flush said container withsaid medium.
 3. The method as in claim 2 wherein said flowing stepcomprises using a medium in which said proportion of additive materialtherein is greater than the proportion of said additive material in saidliquid crystal material.
 4. A method of fabricating a liquid crystaldevice comprising a container and a filling including a mixture of aliquid crystal and an additive material, said additive material tendingto be taken up by the walls of said container and thereby depleted fromsaid mixture, said method comprising: flushing said container withadditive material; and then filling said container with said mixture;said flushing step serving to prevent depletion of said additivematerial from said mixture placed in said container by said fillingstep.